This invention reltes to a circuit providing a bypass around a delta-wye transformer in both directions for a balanced phase-to-neutral power line communications signal, while also providing a signal voltage ratio equivalent to the power line voltage ratio of the transformer. The bypass circuit utilizes passive components and is of a design which lends itself particularly to power line communication systems employing carrier frequencies in the 3-10 KHz range.
A balanced phase-to-neutral power line communications system is designed to provide the same signal voltage with respect to neutral on each of the conductors of a three phase line. Such a signal, balanced in each of the three phase conductors, does not significantly couple through a delta-wye transformer. This is because there is no connection to neutral on the delta side of the transformer, and the phase-to-phase signal voltage in the delta windings is designed to be zero. Therefore, in order to transmit the communications signal in a line containing, for example, a delta-wye distribution feeder step-down transformer, a bypass around the transformer must be provided for the signal.
Because it is desirable to be able to transmit the communication signal in either direction along the three phase lines which include the transformer, it is important that the bypass be bidirectional.
On either side of a distribution feeder transformer, there can be customer distribution transformers which provide, for example, a two-line voltage of 120 volts. Obviously, a customer distribution transformer providing 120 volts and located on the higher voltage side of a delta-wye feeder transformer must have a different voltage ratio than a customer distribution transformer providing 120 volts on the lower voltage side of the delta-wye transformer. It is desirable that the communications signals on the two 120-volt customer lines be in the same voltage range. Therefore, it is an advantage if the bypass around the delta-wye transformer provides the same voltage ratio for the communications signal as the delta-wye transformer provides to the power voltage.